1. Field of the Invention
The present invention relates in general to automatic transmissions and more particularly to speed change control devices of such automatic transmissions.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional speed change control device disclosed in Japanese Patent Second Provisional Publication No. 58-27419 will be described.
The speed change control device is designed for an automotive automatic transmission having four forward speeds and one reverse. The speed control device has three shift valves, which are a 1-2 shift valve, a 2-3 shift valve and a 3-4 shift valve. These three shift valves are subjected to a position change due to a hydraulic pressure supplied from first and second solenoid valves. In particular, the hydraulic pressure from the first solenoid valve is applied to the 2-3 shift valve to carry out position change of that shift valve, while the hydraulic pressure from the second solenoid valve is applied to both the 1-2 and 3-4 shift valves. However, the application of hydraulic pressure to these two shift valves from the second solenoid valve does not always induce a change in the position of these two valves. More specifically, when the 2-3 shift valve assumes a second speed position, the hydraulic pressure from the second solenoid valve changes the position of only the 1-2 shift valve. In this case, the position of the 3-4 shift valve is not changed. In other words, when the 2-3 shift valve assumes the second speed position, the hydraulic pressure application to the 3-4 shift valve from the second solenoid valve is made through the 2-3 shift valve, thereby fixing or maintaining the existing position of the 3-4 shift valve. When the 2-3 shift valve assumes a third speed position, the hydraulic pressure from the second solenoid valve changes the position of only the 3-4 shift valve. In this case, the position of the 1-2 shift valve is not changed. That is, when the 2-3 shift valve assumes the third speed position, the hydraulic pressure application to the 1-2 shift valve from the second solenoid valve is made through the 2-3 shift valve thereby to fix or keep the existing position of the 1-2 shift valve. With this arrangement, the 1-2 speed change and the 3-4 speed change are controlled by the second solenoid valve.
However, due to an inevitable increase in size of the 2-3 shift valve, the above-mentioned conventional speed change control device is forced to have a complicated and bulky construction. That is, in addition to its essential function by which the hydraulic pressure application to the 2-3 shift valve is switched, the 2-3 shift valve needs an additional function by which the hydraulic pressure for suppressing the position change of the 1-2 and 3-4 shift valves is outputted at the second and third speed positions of the 2-3 shift valve, respectively. Furthermore, it is necessary to provide fluid passages for fluidly connecting the 2-3 shift valve to both the 1-2 shift valve and the 3-4 shift valve. This also contributes to the bulky construction of the speed change control device.